90 VISION WITH THE COMPOUND MICROSCOPE 



As the amplification increases the over-amplification of the 

 depth dimension presents increasingly unfavourable relation between 

 the depth and width of the object-space accessible to accommodation. 

 When low powers are employed we have relatively great depth of 

 vision, because we have large accommodation-depth ; but as we pass 

 to medium powers, the accommodation-depth diminishes in rapid 

 ratio, becoming equal to only a small depth of focus ; while when 

 the magnifying power is greatly increased the accommodation-depth 

 is a fax-tor of no moment, and we have vision largely, indeed almost 

 wholly, dependent on depth of focus. 



The following table shows the total depth of vision from ten to 

 M.OOO times:- 



It has been pointed out by Abbe that this over-amplification of 

 depth-dimension, though it limits the direct appreciation of solid 

 forms, yet is of great value in extending the indirect recognition of 

 spare relations. When with increase of magnifying power the depth 

 of the image becomes more and more flattened, the images of different 

 planes stand out from each other move perfectly in the same ratio. 

 and in the same degree are clearer and more distinct. With an 

 increase of amplification the microscope acquires increasingly the 

 property of an optical microtome, which presents to the observer's 

 eye sections of a fineness and sharpness which would be impossible 

 to a mechanical section. It enables the observer, by a series of 

 adjustments for consecutive planes, to construe the solid forms of 

 the smallest natural objects with the same certainty as he i> 

 accustomed to see with the naked eye the objects with which it i> 

 concerned. This is a large advantage in the general scientific u-e 

 of the instrument; a greater gain, in fact, than could be expected 

 from the application of stereoscopic observation. 



Stereoscopic Binocular Vision. This subject has been elaborately 

 considered and partially expounded and demonstrated by Professor 

 Abbe ; his exposition differs in some important particulars from 

 that of the original author of this book, but in its present incomplete 



